This invention generally relates to seal structures for Surface Effect Ships and more particularly to a seal structure which includes transverse reinforcing means for reducing weight requirements and the effects of impact loads.
Surface Effect Ships (SES) of the type referred to as Captured Air Bubble (CAB) vessels utilize a cushion of air contained within a plenum chamber beneath the vessel for supporting the vessel above the surface of the water. In marine CAB vessels the plenum chamber is commonly defined as a space between a pair of spaced, depending sidewalls with bow and stern seals extending athwartship between respective fore and aft end portions of the sidewalls. Problems associated with seal structures for CAB vessels include excessive loss of pressurized air from the plenum chamber adjacent the seals; high seal wear rates at the seal-water interface; structural problems caused by snap back and slamming loads of the seals; and high impact and drag forces due to nonresponsive and noncompliant seal structures. Although various types of seal structures, such as the planer, the bag and finger, and the bag and planer seal constructions have been proposed to alleviate these problems, the additional strength and weight requirements for the new generation of large, high speed CAB vessels have precluded the use of many seal designs in such vessels. For example, large CAB vessels generally require large, lightweight and strong seal structures having rapid response characteristics to accommodate rapid variations in the water surface at high speeds. Also, the seal structures must be durable enough to endure impact and dynamic seal weight loads imposed thereon under severe sea state conditions.
It was found, for instance, that when many prior types of seal structures are scaled up to fit large CAB vessels, such seal designs become unduly heavy, ponderous and unresponsive in various sea state conditions. For example, although the planing seal disclosed in U.S. Pat. No. 3,532,180 granted to Ford and Wilson appears to be adequate for small vessels, it is difficult to scale the seal design up to the size required for large CAB vessels. The Ford seal structure includes a membrane and a plurality of flexible rods or spring steel strips incorporated therein for supporting the membrane. However, the rods or steel strips do not lend themselves to scaling up in size since they become much heavier and stiffer as their dimensions are increased, which accordingly reduces the responsive and planing characteristics of the seals.
An example of a proposed planing seal for large CAB vessels is disclosed in U.S. Pat. No. 4,137,987 granted to M. Plackett. The Plackett seal generally comprises a flexible bag structure secured between two spaced sidewalls and a planer seal portion of juxtaposed, elongated planer members connected to the hull and extending beneath the bag structure. The planers are maintained in operative position by a cooperative arrangement of support stays, which extend from the fore end portions of the planers to the hull, and retracting and geometry straps, which extend from the hull and are connected to the aft end of the planers. Thus, in operative position the elongated planers are designed to act as planer surfaces for riding over the water surface. A biasing or equilibrium means for causing the planers to return to a predetermined equilibrium position is provided by the bag structure which, upon inflation, exerts an outward pressure on the stay members. However, large scale seal structures utilizing the planers embodied in the Plackett patent have proved to be rather heavy and subject to severe slamming loads as the planers encounter the waves at high speeds, due to their weight and stiffness characteristics. The heavy weight of the planers also tends to cause the seal structure to be highly stressed under severe operating conditions such as "snap back". Snap back of the planers occurs after the lower portion of the seal is forced toward the hull by a wave which, after the wave passes the seal structure, causes a gap between the lower portion of the seal and the surface of the water. Under the influence of gravity and the cushion pressure, the planers accelerate toward the water until they are restrained or "snatched" by the flexible retracting and geometry support straps. Attempts to strengthen the planers with various reinforcing means has tended to reduce the flexibility and responsive characteristics of the planers which, accordingly, induces larger impact and drag forces on the seal structure.